Method of metal plating aluminum alloys



United States Patent Ofiice 3,404,998 METHOD OF METAL PLATING ALUMINUM ALLOYS Bernard I. Pesetsky, South Charleston, W. Va., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed May 18, 1965, Ser. No. 456,834

Claims. (Cl. 117-62) ABSTRACT OF THE DISCLOSURE A method of metal plating aluminum alloys without permanent loss of strength or hardness due to annealing caused by high plating temperatures. The method comprises decomposition of a bis,(arene)metal compound to form a metal plate on the aluminum alloy followed by solution heat-treatment and precipitation hardening to re store the strength and hardness to substantially their original levels. Both the heating and quenching steps of the solution heat-treatment are carried out under the inert atmosphere of the plating zone so that discoloration problems are effectively avoided.

This invention relates to a process for producing a metal plate on an aluminum alloy substrate. More particularly, this invention relates to a method of metal plating an aluminum alloy comprising vapor phase decomposition of a bis(arene)metal compound followed by solution heattreatment and precipitation hardening.

Formation of an adherent metal plate on a solid platable substrate may be accomplished in a convenient and effective manner by contacting the substrate in an inert atmosphere with a bis(arene)metal compound at a temperature at or above the thermal decomposition temperature of the bis(arene) metal compound. Plating of aluminum alloys by this means to improve their appearance, corrosion resistance, or wear resistance is an important application of this plating technique. However, in many instances, the plating process may detrimentally affect the aluminum alloy. The plating temperature required will be dependent upon the temperature necessary to effect thermal decomposition of the bis(arene)metal compound. With bis(arene)chromium compounds, for example, the plating temperature will ordinarily be at least about 300 C. and preferably at least about 350 C. At such elevated temperatures, the plating process results in annealing of the aluminum alloy and, as a consequence thereof, the alloy is left mechanically weakened and softened. Once plating has been carried out, effecting a change in the strength and hardness of the plated alloy presents a difficult problem. The use of conventional cold working techniques may be precluded by the brittle nature of the plate, as would be the case, for example, with a chromium plate, or by the fact that the change in size and shape of a plated aluminum object that would result from cold working could not be tolerated. In many cases, one objective of plating will be to enhance the appearance of the aluminum alloy, so that any treatment of the plated alloy that results in discoloration of the metal plate would not be permissible. With a chromium plate, for example, contact with trace amounts of oxygen or water vapor at elevated temperatures can lead to severe discoloration so that heat treatment of the plated aluminum alloy in accordance with conventional procedures would not be satisfactory.

It is an object of this invention to provide a process for plating an aluminum alloy with an adherent metal plate. A further object of this invention is to provide a method of producing a metal plated aluminum alloy having strength and hardness substantially equivalent to that 3,404,998 Patented Oct. 8, 1 968 to those skilled in the art in light of the teachings herein set forth.

The present invention provides for metal plating and heat treatment of a precipitation-hardenable aluminum alloy in a single operation whereby a plated alloy of improved strength and hardness is produced without detrimentally affecting the metal plate. In accordance with the invention, the aluminum .alloy is contacted over the area to be plated with a bis(arene)metal compound and then heated at a temperature of at least the decomposition temperature of such compound to effect formation of an adherent metal plate on the alloy. Such contact and heating are effected in a plating zone under an inert atmosphere. After formation of the metal plate is completed, the alloy is then subjected to solution heat-treatment, i.e., maintained for a sufiicient period at a'temperature which results in conversion of the aluminum alloy to a substantially homogeneous solid solution and then quenched to form a supersaturated solid solution. It is an important feature of the invention that the solution heat-treatment, including both the heating and quenching operations, is effected without removing the metal plated alloy from the inert atmosphere of the plating zone. Following the solution heat-treatment, the metal plated alloy is maintained at a suitable temperature for a period suflicient to effect precipitation hardening. The final product is an aluminum alloy which is coated with an adherent metal plate that is completely free of discoloration and which possesses strength and hardness substantially equivalent to that of an unplated alloy of the same composition. As will be apparent to one skilled in the plating art, the present invention is of particular utility where the temperatures encountered in the plating step result in annealing of the aluminum alloy, or where it is desired to plate an aluminum alloy that is already in an annealed state but to produce a plated alloy having the strength and hardness of the nonannealed alloy, or where discoloration of the metal plate presents a problem.

The method of this invention may be advantageously employed with any precipitation-hardenable aluminum alloy, i.e., any aluminum alloy capable of undergoing imminum Handbook, Aluminum Company of America, 1962).

The bis(arene)metal compounds contemplated by this invention are organometallic compounds in which the arene groups are uncharged aromatic hydrocarbons containing either an isolated benzene ring or an aryl-substituted benzene structure. As employed herein, the term isolated benzene ring is intended to define a benzene ring as contained in benzene itself, or in a fused ring polycyclic aromatic hydrocarbon containing a benzene ring, wherein, by the Kekule formulation, any double bond external to the benzene carbon ring is removed from carbon atoms. Thus, benzene, indane, tetrahydronaphthalene, 9,10-dihydroanthracene, 9,IO-dihydrophenanthrene, alkyl-substituted benzenes such as toluene, ethylbenzene, octylbenzene, etc., and alkenyl-substituted benzenes in which double bonds external to the benzene ring are separated therefrom by at least two carbon atoms, such-as allylbenzene, etc., are examples of aromatic hydrocarbons containing an isolated benzene ring. By contrast, naphthalene, indene, anthracene, phenanthrene, and styrene are examples of aromatic hydrocarbons which do not contain an isolated benzene ring. 1

As also mentioned above, a class of aromatic hydrocarbons which do not contain an isolated benzene ring, namely aryl-substituted benzenes, also form bis(arene) metal compounds which are useful in the plating process of this invention. Examples of such aryl-substituted benzenes are polyphenyls, alkyl-substituted polyphenyls, such as p-isopropyldiphenyl and p,p'-dimethyldiphenyl, phenylanthracene and phenylphenanthrene.

The bis(arene)metal compounds contemplated by this invention may, from the point of view of their organic moiety, be characterized as addition compounds in contrast to organometallic substitution compounds, wherein hydrogen or another substituent of the organic nucleus is substituted or removed in forming the organometallic compound. Thus, the bis(arene)metal compounds are to be distinguished from the organometallic compounds formed by the chemical bonding of a cyclopentadienyl radical with an element (Fischer and Pfab, Zeit. fiir Naturforschung, 7b, page 377, (1952)), and from phenyl mercury compounds, e.g., phenylmercuric acetate (U.S. Patent 2,502,222). Formation of such substitution compounds, it is to be noted, involves elimination of hydrogen from the cyclopentadiene or benzene nucleus. In the case of the bis(arene)metal compounds of the present invention, the chemical union of the metal with the aromatic hydrocarbon does not involve the elimination of hydrogen or any other substituent of the organic nucleus. The bis(arene)metal compounds may, therefore, be regarded as the addition products of metals with aromatic hydrocarbons.

The bis(arene)metal compounds contemplated by this invention can be represented more clearly by the formula (Ar) M, wherein Ar designates either an aromatic hydrocarbon containing an isolated benzene ring, or an arylsubstituted benzene, as hereinabove described, and M is vanadium, niobium, tantalum, chromium, molybdenum or tungsten. The bis(arene)metal compounds can also have mixed Ar substituents, as more specifically represented by the formula (Ar) (Ar') M wherein Ar and Ar independently designate members of the same class of aromatic hydrocarbons. The exact nature of the bond between the aromatic hydrocarbon moiety and the metal is unknown. However, it is known that the isolated benzene ring or the benzene ring of the aryl-substituted benzene is complexed to the metal.

The bis(arene)metal compounds contemplated by this invention include, by way of illustration,

bis (benzene) chromium, bis (benzene vanadium, bis (benzene) molybdenum, bis (benzene tungsten,

bis (benzene niobium,

bis (benzene tantalum,

' bis (tetrahydronaphthalene chromum,

bis (toluene chromium,

bis (toluene molybdenum, bis(mesitylene chromium,

bis (hexamethylbenzene) chromium, bis ortho-xylene chromium,

bis meta-xylene chromium,

bis para-xylene chromium,

is( l n ckhr mi m bis(benzene) tetrahydronaphthalene)chromium, bis(diphenyl)chromium, bis(cumene)molybdenum, bis(cumene)chromium,

and the like. The preferred bis(arene)metal compounds are those in which the arene moiety is either benzene or a lower alkyl-substituted benzene in which the lower alkyl substituent(s) contain from 1 to about 8, and preferably from 1 to about 4 carbon atoms. 7

The bis(arene)metal compounds can be prepared by reacting an anhydrousmetal salt, such as a-metalhalide, with an aromatic hydrocarbon containing at least one isolated benzene ring, or with an aryl-substituted benzene, in the presence of an anhydrous aluminum halide and a reducing agent. The bis(arene)metal compounds and methods for their production are described more fully in United States Patent 2,953,586 and such description is incorporated herein by reference.

The plating step of the method of this invention involves contacting the aluminum alloy over the area to be plated with the bis(arene)metal compound and heating the bis(arene)metal compound in contact with the alloy at a temperature of at least the decomposition temperature of the -bis(arene)metal compound. The plating is carried out in an inert atmosphere, i.e., in the absence of oxygen or oxygen-containing substances which react with the bis(arene)metal compound, or with the resulting metal plate itself, to form metal oxides which contaminate the plate, and a substantially pure, ordinarily uniform metal plate is thereby produced on the aluminum alloy. A suitable inert atmosphere is, for example, an atmosphere of argon, nitrogen, helium or hydrogen.

The plating temperature employed is a temperature of at least the decomposition temperature of the bis(arene) metal compound, but is preferably somewhat above such temperature, for example, about seventy five degrees above the decomposition temperature. The maximum plating temperature is, for practical purposes, determined by the decomposition (cracking) temperature of the arene moiety of the his (arene)metal compound. Typically, plating temperatures in the range of from about C. to about 450 C. are employed. The contact period may vary from about 10 minutes, or less, to about 3 hours, or more, with thicker plates ordinarily being produced as the contact period is extended.

Plating of the aluminum alloy can be effected in any suitable manner. For example, the bis(arene)metal compound may be vaporized in a stream of inert. gas and passed at atmospheric pressure over the aluminum alloy while maintaining the alloy at a temperature equal to or greater than the decomposition temperature ofthe bis- (arene)metal compound. Heating of the aluminum alloy may be accomplished by such methods as infrared heating, induction heating, resistance heating, laser beam heating, etc. v i

. Following application of the metal plate as described hereinabove, the plated alloy is subjected to solution heattreatment comprising a heating step, and a quenching step. As hereinbefore disclosed, the heating and quenching operations are carried out in the plating zone and thus under the inert atmosphere employed in the plating step. By this means, the heat-treatment of the plated alloy is accomplished without adversely affecting the metal plate. Trace quantities of the bis(arene)metal compound remaining in the plating zone while the heat-treatment is effected tend to act as getters so that an extremely inert atmosphere is attained and undesirable discoloration of the,,metal plate is substantially totally avoided. An additional advantage accrues in thateconomy in heat input is achieved because the heat stored in the alloy, at the end of the plating step is utilized in the subsequent heattreatment.

The solution heat-treatment is carried out by heating the metal plated alloy to at least the solution heat-treatmen temperature for the particular alloy involved and '5 maintaining the alloy at this temperature for a period sufiicient to establish a substantially homogeneous solid solution of the alloying elements in the aluminum. The

solution heat-treatment temperature is dependent upon the particular alloy involved and the treating time for optimum results will also vary from alloy to alloy. As a minimum, the treating time must be sufiicient that every portion of the alloy to be hardened will be heated to at least the solution heat-treatment temperature. Ordinarily, the solution heat-treatment temperature will be .above the plating temperature, for example, a temperature in the range from about 425 C. to about 600 C. The treating time employed may be as brief as several minutes or as long as several hours.

After the .alloy has been maintained at or above solution heat-treatment temperature for a suflicient period, it is quenched, i.e., rapidly cooled, to effect formation of a supersaturated solid solution. In accordance with the present invention, quenching is carried out Within the plating zone and thus under an inert atmosphere. Quenching may be accomplished by spraying the plated alloy with water or oil or other suitable quenching liquid, or by dipping the plated alloy in the quenching agent. It is also feasible, in certain instances, to effect quenching by use of a high velocity stream of cooled inert gas. Although the quenching can be carried out in any suitable manner, it is preferred that the alloy be reduced from solution heat-treatment temperature to a temperature at least below about 100 C. in a period of about 40 seconds or less.

As a final step in the method of this invention, the metal plated aluminum alloy undergoes precipitation 6 and is readily adaptable for use with any precipitationhardenable aluminum alloy.

The following examples are illustrative of the practice of the invention.

Samples of 2024 wrought aluminum alloy in the form of 6-inch long by As-inch thick tensile dummies were suspended in a plating chamber and heated to a plating temperature of 425 C. by infrared lamps through infrared-permeable windows in the plating chamber. Then dicumenechromium vaporized in hot nitrogen was directed into the plating chamber and into contact with the suspended specimens. The gas stream contained approximately 1 mole percent dicumenechromium and a flow rate of 8 standard cubic feet per hour of nitrogen was maintained. Exhaust gases were vented from the plating chamber. Plating occurred at a rate of about 0.2 to 0.3 mil per hour and when the desired plate thickness was attained the flow of dicumenechromium was stopped but the nitrogen flow was maintained. After completion of the plating step, the temperature of the specimen was increased to 495 C. and maintained at this level for 10 minutes and then quenching was effected by spraying the specimen with cold water to reduce its temperature to about C. in 6 to 40 seconds. The specimen was then aged at the conditions of time and temperature indicated below to effect precipitation hardening. For convenience results are tabulated in Table I below. For comparison purposes, data are included for unplated specimens of the same alloy and for specimens that were chromium plated but not subjected to the solution heat-treatment or precipitation-hardening steps.

TABLE I Specimen Plate Solution Precipitation Ultimate Yield Hardness Appearance No. Plating thickness heat-treatment hardening strength strength (Rockwell rating 4 (mil) (p.s.i.) (p.s.i.) 1 )3") 8 1 None None. None 65, 900 54, 800 76 2 d0 do- 66, 300 45, 200 76 3 d do im-.. 66,000 45, 200 76 4 Digulnanechromium at 0. 51 do do 47, 750 23, 350 52 5 5 do. 0. 61 49, 350 25, 200 10 a do 0. 79 -do 49, 200 23, 500 45 10 7 do 0. 53 24 lours at room remper- 60, 700 39, 850 74 9. 0

. a ure. 8 do- 0. 61 dn 59, 200 37, 700 75 10 9 dn 0,36 12 hours at 195 0--.- 64, 900 46, 850 80 9. 5 1n rln 1. 30 53, 500 45, 050 79 9.1 11. do 1. 26 68, 100 48, 050 78 10 fiisTM E8-61T, ASTM Standards, Part 3, Metals Test Methods" 1 I ASTM E8-61T, ASTM Standards, Part 3, Metals Test Methods 5 611\)S TM 1118-61, ASTM Standards, Part 3, "Metals Test Methods" treatment to efiect precipitation hardening. Such treatment comprises maintaining the alloy at a suitable temperature for a suitable period so that precipitation with resultant rehardening and restrengthening of the alloy occurs. The optimum combinationof time and temperature ofprecipitation to accomplish maximum improvement'in properties will vary for each aluminum alloy. In some instances, the precipitation hardening is conducted at room temperature while in other instances it will be desirable to employ elevated temperatures, of up to as high as about 200 C. (artificial aging). The time required to effect precipitation hardening will vary widely from as little as about 3 hours, or less, to as much as about 5 days, or more. Generally, a decrease in the aging temperature requires a corresponding increase in the aging time.

As hereinbefore disclosed, the method of this invention permits the production of metal plated aluminum alloys having mechanical strength and hardness substantially equal to that of the nonannealed aluminum alloy. It is particularly advantageous in that it eliminates undesired discoloration of the plate and is applicable to plating of objects in their finished form. Moreover, the method disclosed is economical and easy to carry out 4 After 24-hour CASS test according to ASTM B368-61'I with rating in accordance with method of ASTM Committee B-8 described in Electroplating Engineering Handbook, 2nd Edition, Reinhold Publishing Corp., New York, 1962, pp. 31%324.

The data presented in Table I above demonstrate that the metal plating step of the present invention results in a substantial decrease in ultimate strength;,; yield strength and hardness, but that operation in accordance with the method of the invention whereby solution heattreatment and precipitation hardening are effected provides a plated alloy with substantially equivalent strength and hardness properties to those of the original alloy. Moreover, the data show that the solution heat-treatment and precipitation-hardening steps are accomplished with little or no adverse eifect on the appearance of the metal plate. 1

Although the invention has been illustrated by the preceding examples, it is not to be construed as limited to the materials employed therein, but rather the invention encompasses the generic area as hereinbefore disclosed. Various modifications and embodiments of this invention can be made without departing from the spirit and scope thereof.

What is claimed is:

1. A method of metal plating a precipitation-hardenable aluminum alloy which comprises:

(1) contacting said alloy over the area to be plated in a plating zone under an inert atmosphere with a vaporized bis(arene)metal compound of the formula (Ar) M, wherein Ar designates an organic hydrocarbon selected from the group consisting of aromatic hydrocarbons containing an isolated benzene ring and aryl-substituted benzenes and M designates a metal selected from the group consisting of vanadium, niobium, tantalum, chromium, molybdenum and tungsten;

(2) heating said alloy in said plating zone in contact with said bis(arene)metal compound at a temperature of at least the decomposition temperature of said bis(arene)metal compound to form a metal plate on said alloy;

(3) further heating said alloy in said plating zone after deposition of the metal plate is effected at solution heat-treatment temperature for a period sufficient to form a substantially homogeneous solid solution;

(4) rapidly cooling said alloy in said plating zone to form a supersaturated solid solution; 1

() hOlding said alloy at a temperature and for a period of time sufiicient to effect precipitation hardening; and

(6) recovering said alloy with an adherent metal plate thereon.

2. A method of chromium plating a precipitationhardenable aluminum alloy which comprises:

(1) contacting said alloy over the area to be plated in a plating zone under an inert atmosphere with a vaporized bis(arene)chromium compound of the formula (Ar) Cr, wherein Ar designates a lower alkyl-substituted benzene in which the lower alkyl substituents contain from 1 to about 4 carbon atoms;

(2) heating said alloy in said plating zone in contact with said bis(arene)chromium compound at a temperature of at least the decomposition temperature of said bis(arene)chromium compound to form a chromium plate on said alloy;

(3) further heating said alloy in said plating zone after deposition of the chromium plate is effected at solution heat-treatment temperature for a period sufficient to form a substantially homogeneous solid solution;

(4) rapidly cooling said alloy in said plating zone to form a supersaturated solid solution;

(5) holding said alloy at a temperature and for a period of time sufficient to effect precipitation hardening; and

(6) recovering said alloy with an adherent chromium plate thereon.

3. A method of chromium plating a precipitation-hardenable aluminum alloy which comprises:

(1) contacting said alloy over the area to be plated in a plating zone under an inert atmosphere with vaporized bis (benzene) chromium;

(2) heating said alloy in said plating zone in contact with said his (benzene)chromium at a temperature of at least the decomposition temperature of said his (benzene)chromium to form a chromium plate-on said alloy;

(3) further heating said alloy in said plating zone after deposition of the chromium plate is effected at solu- 8 tion heat-treatment temperature for a period sufficient to form a substantially homogeneous solid solution;

(4) rapidly cooling said alloy in said plating zone to form a supersaturated solid solution;

(5) holding said alloy at a temperature and for a period of time sufficient to effect precipitation hardening; and

(6) recovering said alloy with an adherent chromium plate thereon.

4. A method of chromium plating a precipitation-hardcnable aluminum alloy which comprises:

(1) contacting said alloy over the area to be plated in a plating zone under an inert atmosphere with vaporized dicumenechromium;

(2) heating said alloy in said plating zone in contact with said dicumenechromium at a temperature of at least the decomposition temperature of said dicumenechromium to form a chromium plate on said alloy;

(3) further heating said alloy in said plating zone after deposition of the chromium plate is effected at a solution heat-treatment temperature for a period sufficient to form a substantially homogeneous solid solution;

(4) rapidly cooling said alloy in said plating zone to form a supersaturated solid solution;

(5) holding said alloy at a temperature and for a period of time sufficient to effect precipitation hardening; and

(6) recovering said alloy with an adherent chromium plate thereon.

5. The method according to claim 1 wherein the inert atmosphere is an atmosphere of nitrogen gas.

6. The method according to claim 1 wherein control of the temperature of the alloy is accomplished by infrared heating.

7. The method according to claim 1 wherein control of the temperature of the alloy is accomplished by induction heating.

8. The method according to claim 1 wherein'the rapid cooling of the alloy is effected by dipping the alloy in a liquid quenching agent. 7

9. The method according to claim-1" wherein the rapid cooling of the alloy is effected by spraying the alloy with a liquid quenching agent.

10. The method according to claim 1 .wherein the rapid cooling of'the alloy is effected by spraying the alloy with ALFRED L. LEAVITI, Primary Examiner, A. GOLIAN, Assistant Examiner. 

